Organic write-once optical recording medium with surface plasmon super-resolution layer

ABSTRACT

An organic write-once optical recording medium includes a surface plasmon super-resolution layer. The surface plasmon super-resolution layer is a three-layer structure including a first dielectric layer, a second dielectric layer, and a metal layer sandwiched between said first dielectric layer and said second dielectric layer. The metal layer with a certain thickness performs the surface plasmon effect when a laser beam with a suitable wavelength irradiates thereon. By the design and arrangement of the surface plasmon super-resolution layer, the small size of information-carrying pits and the recording marks in the range of around 100 nm is accessible. As a result, the super-resolution without the limit of the optical diffraction is achieved.

FIELD OF THE INVENTION

The present invention relates to an organic write-once optical recordingmedium, and more particularly, to an organic write-once opticalrecording medium with surface plasmon super-resolution layer, whereinthe surface plasmon super-resolution layer is incorporated to enhancethe near-field intensity of a light beam and obtain a smaller readingspot size, thereby achieving a function of the organic write-onceoptical recording at high density. The surface plasmon super-resolutionlayer is applicable for the optical recording media with multi-layersfor recording information.

BACKGROUND OF THE INVENTION

In 1980, the Philips Company proposed a recording medium having atransparent substrate and a plurality of data pits, and accessed byirradiating a laser beam through the transparent substrate. Therecording medium is referred to as a compact disk (CD) and becomes moreand more prevailing ever since. Moreover, many types of CD, for example,CD, CD-G; CD-I, photo-CD, VCD, CD-R and CD-RW, have been suggested andrealized. In 1995, due to the increasing demand for larger amount ofinformation, a more advanced type of optical disk, digital versatiledisk (DVD), is proposed to provide 4.7 GB data on a single sided diskwith a diameter of 12 cm.

The density enhancement of DVD can be manifested from the followingcomparison. The CD family employs a laser beam having a wavelength of780 nm and a lens with the N.A. (Numeral Aperture)=0.45 to access thedata stored therein. The pitch between two adjacent tracks of the CD isabout 1.6 μm. On the other hand, the DVD family employs a laser beamhaving a wavelength of 650 nm and a lens with the N.A.=0.6 to access thedata stored therein. The pitch between two adjacent tracks of the DVD isabout 0.74 μm.

Besides the pre-recording type optical media, multi-rewritable opticaldisks are also developed for the demand for the storage and modificationof the information. Most of them employ phase-charge materials. As thedevelopment of various phase-change materials and the success of thedirect writing technique, the multi-rewritable and erasable opticalrecording media are merchandised. For example, PD and CD-RW (650 MB) areproduced in 1997, and 2.6 GB DVD RAM by DVD union, 3.0 GB DVD+RW disk byPhilips.

As to the mass production of the organic write-once optical disk, thedata to be recorded in the optical disk are first processed throughscrambling, interleave and then encoded by EDC and ECC coding. Then theencoded data is transferred to a stamper, which has pit region to recordthe data. By using the stamper, a substrate with data-recording layercan be mass produced by mold ejection. Thereafter, the substrate iscoated with a lower dielectric layer, a recording layer, an upperdielectric layer and a reflection layer by sputtering. Finally, aprotective layer is applied thereon by spin coating. The user can use alaser beam to form marks on the recording layer by adjusting the powerof the laser beam. The recording layer employs materials with reversiblephase-change ability such that the optical media can be read and writtenrepeatedly. To form pits with a smaller pitch, the wavelength of thelaser beam can be reduced by using the blue light or UV light.Alternatively, the N.A. of the lens system can be further increased soas to increase the data density for the same size of a disk.

Besides, the recording density can be increased by using a moreefficient encoding scheme, reducing the size of the pit and track pitch,or using multilayer techniques.

However, there are still several considerable problems present in theabove approaches. The efficiency of the encoding scheme is limited bythe requirement of the error detection and correction code. The scaledown of the pit and track pitch is limited by the resolution of theoptical instrument. The employment of a shorter wavelength is influencedby the optical response of the material used, and the stability and costof a semiconductor laser. The use of a larger N.A. value is inherentlylimited according to the optics theory and the manufacturing ability.The use of multi-layer techniques is effected by the reading andrecording reliability. Finally, the optical diffraction limit playsessential role in the above-mentioned limitation of the size of the pitand track pitch.

Recently, the optical near-field technique becomes more attractive tothe researcher involved the development of the optical recording medium.For example, it is reported that the pit pitch of 40 nm˜80 nm can berealized by the near-field probe optical recording. The pit pitch can befurther reduced to 35 nm by using super-resolution N.A. with a solidimmersion lens. However, in the above technique, there is a practicalproblem for the design of the optical probe due to the requirement ofcontrolling the distance between the probe and the reading surface ofthe optical medium through the shear force feedback control of theprobe. Moreover, the probe system for this near-field technique is notcompatible with the present system, and it is possible to damage andbreak the probe.

Moreover, a super-resolution structure is proposed wherein thenear-field effect is realized by the special design of the multi-layerbased on the nonlinear optics mechanism other than by the probe.

However, there are some disadvantages present in the above method. Asshown in FIG. 1, the accessing to the high density optical recordingmedium is conducted by the near-field probe. In this method, it isnecessary to maintain a constant distance about 100 nm between the probeand the surface of the disk to achieve the effect of the opticalnear-field. Due to the maintenance of the constant distance, it isnecessary to design a control system to control the dynamic shear forcegenerated by the air flow between the probe and the surface of the disk.As a result, the design is more complicated and is not compatible withthe present reading and writing system. Moreover, in order to controlthe distance, the surface of the disk is usually scratched or the probeis broken resulting in the malfunction of the reading and writingsystem.

For the super-resolution structure, as shown in FIG. 2, an opticalnonlinear film having the thickness of 15 nm is incorporated into thedisk structure thereby conducting the control of the optical near-fieldeffect. The super-resolution structure is easy to implement because theoptical nonlinear film may be formed during the manufacturing process ofthe disk. Furthermore, similar to the present technology about theoptical disk player, it is unnecessary to maintain a minute distancebetween the optical accessing head and the surface of the disk. As aresult, the object of the high density recording is easily achieved byusing this technique. However, the super-resolution effect is notgenerated by the nonlinear optical property of the super-resolutionstructure and the optical nonlinear film is not made of a dielectricmaterial. Therefore, the physical mechanism of the super-resolutioneffect should be clearly exploited such that other possible materialsand structures to generate super-resolution effect are developed.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an organicwrite-once optical recording medium wherein a surface plasmon inducedlayer is incorporated to enhance the field strength of the near-fieldoptical beam and obtain a smaller reading spot size, thereby providing ahigh-density optical recording effect.

According to the present invention, a high density organic write-onceoptical recording medium comprising: a transparent substrate; an organicwrite-once recording layer formed on the transparent substrate; a firstdielectric layer formed on the organic write-once recording layer; ametal layer formed on the first dielectric layer; a second dielectriclayer formed on the metal layer; and a UV coating layer formed on thesecond dielectric layer, thereby a surface plasmon is generated in theinterface between the second dielectric layer and the metal layer when alaser beam with a wavelength in the range form 300 nm to 800 nmirradiates toward the UV coating layer, and obtains the enhancementeffect of the near-field intensity so as to achieve a high resolutionfor distinguishing minute pits.

BRIEF DESCRIPTION OF DRAWINGS

The above-mentioned and other objects, features, and advantages of thepresent invention will be more clearly understood from the followingdetailed description in conjunction with the appended drawings, inwhich:

FIG. 1 is a cross sectional view explaining the accessing process to thehigh-density optical data by a near-field probe;

FIG. 2 is a cross sectional view explaining a super-resolution structureusing a nonlinear optical film with a thickness of 15 nm;

FIG. 3 is a cross sectional view showing the optical configuration forgenerating the surface plasmon by a Sb thin film;

FIG. 4 is a cross sectional view showing a total internal reflectionoccurred in the Sb thin film shown in FIG. 3;

FIG. 5 shows the transmissivity of a P-polarized wave through a Sb filmwith respect to the thickness of the Sb film and the incident angle ofthe P-polarized wave;

FIG. 6 shows the transmissivity of a P-polarized wave through a Sb filmhaving a thickness of 15 nm;

FIG. 7 shows the transmissivity of an S-polarized wave through a Sb filmwith respect to the thickness of the Sb film and the incident angle ofthe S-polarized wave;

FIG. 8 shows the transmissivity of an S-polarized wave through a Sb filmhaving a thickness of 15 nm;

FIG. 9 shows the simulation result of transmissivity of a laser beamwith a wavelength of 635 nm through a Sb film having a thickness of 15nm;

FIG. 10 shows the diameter of a light spot generated by the surfaceplasmon effect according to the present invention;

FIG. 11 is a cross sectional view showing a first embodiment accordingto the present invention;

FIG. 12 is a cross sectional view showing a second embodiment accordingto the present invention;

FIG. 13 is a cross sectional view showing a third embodiment accordingto the present invention; and

FIG. 14 is a cross sectional view showing a fourth embodiment accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The surface plasmon is a quantized oscillation of collective electronsin an interface between a metal and a dielectric. It is associated withan electromagnetic field, which is not propagating but is evanescent atthe metal surface. The surface plasmon is excited upon the surface of ametal film irradiated by photons at an incident angle that satisfies thedispersion matching condition between the photon and the surface plasmonin the metal.

As shown in FIG. 3, this figure shows the optical configuration forgenerating surface plasmon on Sb thin film, wherein the laser beam isirradiated on the Sb thin film with a specific incident angle θ_(sp). Asshown in FIG. 4, the refractive index n1 of the metallic layer is largerthan the index n2 of the lower dielectric layer (the inner totalreflection may occur in this structure). When the incident angle θ_(sp)of the laser beam is such that the dispersion matching condition can besatisfied, the quantized oscillation of collective electrons in aplasmon surface is occurred.

According to the condition of standing wave: L=m(λ/2)=d/cosθ_(sp), thequantized oscillation of plasmon is occurred when the thickness of themetal d=m(λ/2)cos_(sp). As to the dispersion matching condition of thesurface plasmon, the propagating constant is${k_{sp} = {\frac{\omega}{c}\sqrt{\frac{{\varepsilon_{1}(\omega)}{\varepsilon_{2}(\omega)}}{{\varepsilon_{1}(\omega)} + {\varepsilon_{2}(\omega)}}}}},$

∈_((Re))=n²−k², and ∈_((Im))=2nk, wherein is the angular frequency, C isthe speed of light in vacuum ∈₁(ω), ∈₂(ω) is the dielectric constants inair and metal, respectively. In generally,k_(sp)(ω)/k_(light)(ω)≡1.05˜1.10 and as to Sb, ∈_((Re))=22.36,∈_((Im))=35.21, therefore, k_(sp)(ω)/k_(light)(ω)≡1.02. Therefore, theincident laser beam will excite surface plasmon on Sb film by choosingobject lens with proper N.A.

The incident laser beam can be polarized to an S-wave and a P-wave.FIGS. 5 to 8 show the numeral simulation result for the Sb filmirradiated by a laser beam. FIG. 5 shows the transmissivity of aP-polarization wave with respect to the thickness of the Sb film and theincident angle of the P-polarized wave. FIG. 6 shows the transmissivityof a p-polarization wave through a Sb film having a thickness of 15 nm.FIG. 7 shows the transmissivity of an S-polarized wave through a Sb filmwith respect to the thickness of the Sb film and the incident angle ofthe S-polarized wave. FIG. 8 shows the transmissivity of a S-polarizedwave through a Sb film having a thickness of 15 nm. As can be seenthrough FIGS. 5 to 8, the transmissivity of the P-polarization waveincreases along with the decrease of the thickness of the Sb film whilethe transmissivity of the S-polarization wave is always rather small andalmost constant.

As the result of the numerical simulation analysis, FIG. 9 shows thedistribution of the transmissivity for a Sb film having a thickness of15 nm irradiated by a laser beam with λ=635 nm. The near-field intensitygenerated by the surface plasmon polariton excitation is enhanced 300times higher than that generated by a structure without the surfaceplasmon effect. FIG. 10 shows the intensity profile of the near-fieldlight according to the present invention with a maximum intensity of I₀.The size of the optical field with the intensity exceeding I₀/e2 isdefined as the spot size of the incident laser beam. The incident laserbeam causes the recording material subjected to a phase change. However,the size of the phase-change area of the recording material is differentfrom the spot size of the incident laser beam. The size of thephase-change area is determined by a threshold intensity I_(threshold).The recording material is subjected to the phase change only if theintensity of the incident laser beam is larger than the thresholdintensity I_(threshold), Moreover, the threshold power may be differentwith respect to different phase change materials. Therefore, the surfaceplasmon control layer according to the present invention can be used inthe application of a multi-layer disk.

The several preferred embodiments according to the present inventionwill be described in detail with reference to FIGS. 11 to 14. Thepresent invention is applicable to various organic write-once opticalrecording media. FIG. 11 shows the first embodiment according to thepresent invention. The organic write-once optical recording mediumaccording to the first embodiment comprises a transparent PC(polycarbonate) substrate 10, a reflective layer 12 formed on thetransparent PC substrate 10, an organic dye layer 16 formed on thereflective layer 12, a first dielectric layer 18 made of SiN and with athickness of 170 nm formed on the organic dye layer 16, a metal layer 20made of Sb and with a thickness of 15 nm formed on the first dielectriclayer 18, a second dielectric layer 22 made of SiN and with a thicknessof 20 nm formed on the metal layer 20, and a UV (ultraviolet) coatinglayer 24 formed on the second dielectric layer 22. The reading laserlight with a wavelength in the range from 300 nm to 800 nm irradiatestoward the side of the UV coating layer 24 and is reflected from thesame side. The reflected light is received by a photo-detector (notshown) to access the data recorded in the organic dye layer 16.

FIG. 12 shows the second embodiment according to the present invention.The organic write-once optical recording medium according to the secondembodiment comprises a transparent PC substrate 10, a second dielectriclayer 22 made of SiN and with a thickness of 20 nm formed on thetransparent PC substrate 10, a metal layer 20 made of Sb and with athickness of 15 nm formed on the second dielectric layer 18, a firstdielectric layer 22 made of SiN and with a thickness of 170 nm formed onthe metal layer 20, an organic dye layer 16 formed on the firstdielectric layer 18, a reflective layer 12 formed on the organic dyelayer 16, and a UV coating layer 24 formed on the reflective layer 12.The reading laser light with a wavelength in the range from 300 nm to800 nm irradiates toward the side of the transparent PC substrate 10 andis reflected from the same side. The reflected light is received by aphoto-detector (not shown) to access the data recorded in the organicdye layer 16.

The embodiments shown in FIGS. 11 and 12 are reflection type disks. Onthe other hand, the embodiments shown in FIGS. 13 and 14 aretransmission type disks. FIG. 13 shows the third embodiment according tothe present invention. The organic write-once optical recording mediumaccording to the third embodiment comprises a transparent PC substrate10, a second dielectric layer 22 made of SiN and with a thickness of 20nm formed on the transparent PC substrate 10, a metal layer 20 made ofSb and with a thickness of 15 nm formed on the second dielectric layer22, a first dielectric layer 18 made of SiN and with a thickness of 170nm formed on the metal layer 20, an organic dye layer 16 formed on thefirst dielectric layer 18, and a UV coating layer 24 formed on theorganic dye layer 16. The optical recording medium according to thethird embodiment does not comprise a reflective layer. The reading laserlight with a wavelength in the range from 300 nm to 800 nm irradiatestoward the side of the PC substrate 10 and is received by aphoto-detector (not shown) arranged on the opposite side (i.e. the sideof the UV coating layer 24).

FIG. 14 shows the fourth embodiment according to the present invention.The organic write-once optical recording medium according to the fourthembodiment comprises a transparent PC substrate 10, an organic dye layer16 formed on the transparent PC substrate 10, a first dielectric layer18 made of SiN and with a thickness of 170 nm formed on the organic dyelayer 16, a metal layer 20 made of Sb and with a thickness of 15 nmformed on the first dielectric layer 18, a second dielectric layer 22made of SiN and with a thickness of 20 nm formed on the metal layer 20,and a UV coating layer 24 formed on the second dielectric layer 22. Theoptical recording medium according to the fourth embodiment does notcomprise a reflective layer. The reading laser light with a wavelengthin the range from 300 nm to 800 nm irradiates toward the side of the UVcoating layer 24 and is received by a photo-detector (not shown)arranged on the opposite side (i.e. the side of the PC substrate 10).

Accordingly, the present invention has the following advantages.

1. The resolution is not limited by the optical diffraction and nospecial instrument is required, so the present invention is applicableto accessing by the laser beam with various wavelengths.

2. The near-field intensity of the incident laser beam is enhanced so asto obtain a higher resolution for minute pits.

3. The accessing of the optical recording medium with the surfaceplasmon super-resolution structure does not involve a specialinstrument; therefore it is compatible with the existing accessingsystem.

4. The surface plasmon super-resolution structure is applicable to theaccessing of the minute data pit, thereby achieving a higher datadensity.

While the invention has been described by way of example and in terms ofthe preferred embodiment, it is to be understood that the invention isnot limited to the disclosed embodiment. To the contrary, it is intendedto cover various modifications and similar arrangements as would beapparent to those skilled in the art. Therefore, the scope of theappended claims should be accorded the broadest interpretation so as toencompass all such modifications and similar arrangements.

For example, the first and second dielectric layers, the organicrecording layer, and the reflective layer can be of multi-layerstructures.

What is claimed is:
 1. A high density organic write-once opticalrecording medium comprising: a transparent substrate; an organicwrite-once recording layer formed on said transparent substrate; a firstdielectric layer formed on said organic write-once recording layer; ametal layer formed on said first dielectric layer; a second dielectriclayer formed on said metal layer; and a UV coating layer formed on saidsecond dielectric layer, thereby a surface plasmon is generated in theinterface between said second dielectric layer and said metal layer whena laser beam with a wavelength in the range form 300 nm to 800 nmirradiates toward said UV coating layer, and obtains the enhancementeffect of the near-field intensity so as to achieve a high resolutionfor distinguishing minute pits.
 2. A high-density organic write-onceoptical recording medium according to claim 1, further comprising areflective layer sandwiched between said transparent substrate and saidorganic write-once recording layer.
 3. A high-density organic write-onceoptical recording medium according to claim 2, wherein said firstdielectric layer is made of SiN having a thickness of 170 nm.
 4. Ahigh-density organic write-once optical recording medium according toclaim 3, wherein said metal layer is made of Sb having a thickness of 15nm.
 5. A high-density organic write-once optical recording mediumaccording to claim 4, wherein said second dielectric layer is made ofSiN having a thickness of 20 nm.